Yellowfin Tuna

This is a large torpedo-shaped fish, with a round body that is metallic blue-green above and silvery white below, with lots of light vertical lines. It has sturdy fins, and a series of bright yellow finlets between the dorsal and anal fins and the crescent caudal (tail) fin. This migratory fish prefers the warm surface water where the younger ones school in big groups, or just below where they can catch smaller fish under drift wood or seagrass. It is popular with commercial fisheries as it grows larger than 100 inches long and more than 400 pounds.

Importance to Humans

Yellowfin are a popular target for commercial fisheries. In the U.S., yellowfin catches have grown to nearly 45% of the U.S. North Atlantic tuna catch. At the surface, they are primarily caught by purse seine. A purse-seine vessel first encircles a school with a large net. The bottom of the net is closed off, and the net is pulled upwards and brought aboard the boat, where the catch can be released by reopening the bottom of the net once it is out of the water. The purse-seine method is central to the "dolphin-safe" tuna fishing legislation. In the late 1950s, fishermen in the eastern tropical Pacific Ocean began to exploit the tendency of yellowfin to school with dolphins. When dolphins were spotted on the surface, the fishermen would encircle them with their purse seines, hoping tuna would be schooling just below the surface. Originally, little effort was made to release the dolphins, which were of no commercial value. The dolphins would become entangled in the nets and drown. Hundreds of thousands of dolphins were killed every year by this method. Current fisheries based out of the U.S. and other nations, in concert with conservationists and consumer interest, are now working to reduce or eliminate dolphin by-catch.

There is also a longline fishery targeting deep-swimming yellowfin, based primarily out of Japan, Korea, and Taiwan. While these fisheries operate worldwide, the majority of catches of deep-swimming tuna are made in the western Pacific. In many regions, especially Pacific oceanic islands, yellowfin are still caught by hook and line.

Yellowfin are a sportfishing target in many areas. They are caught in southern California, Baja, Mexico, and Hawaii, as well as along the southeastern U.S. including the Gulf of Mexico. Yellowfin tuna is a primary fish used for canning for U.S. consumption. In Asia, it is especially popular in raw fish dishes, where it is known as ahi.

Conservation

The yellowfin tuna is currently categorized as "Lower Risk/Least Concern" by the World Conservation Union (IUCN) although this classification is considered out of date according to their web site. The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.

Geographical Distribution

World distribution map for the yellowfin tuna

Yellowfin tunas are found worldwide in tropical and subtropical waters, from latitudes of approximately 40°N to 35°S. They are absent in the Mediterranean Sea. The yellowfin tuna is a highly migratory fish. In the Pacific Ocean, however, there is little evidence for long-range north-south or east-west migration. This suggests relatively little genetic exchange between the eastern, central, and western Pacific Ocean and perhaps the development of subspecies.

Habitat

Schooling yellowfin tuna. Photo courtesy NOAA

The yellowfin tuna is an epipelagic, oceanic fish, living above and below the thermocline, at temperatures of 65 to 88°F (18-31°C). It is generally found in the upper 330 feet (100 m) of the water column.

Yellowfin are strong schoolers. Their tendency to school with organisms of the same size is stronger than the tendency to school by species. They often swim in mixed schools of skipjack, bigeye, and other tunas. In the eastern Pacific Ocean, larger yellowfin frequently school in association with dolphins, particularly the spotted dolphin, spinner dolphin, and common dolphin. Such associations with dolphins have not been observed in the rest of the Pacific, the Indian, or the Atlantic Oceans. Yellowfin will commonly school under drifting objects such as driftwood, patches of seagrass, boats, or dead marine mammals. There are many hypotheses addressing the reasons for schooling under such items. Yellowfin may be attracted to the object to feed on smaller prey which are foraging on the structure. The drifting object provides shade and shelter from predators. Yellowfin tuna may utilize the object as a substrate on which to lay their eggs or as a "cleaning station," where parasites are removed by other fishes. Also, the fish may view the object as a "schooling companion".

Yellowfin swimming further from the surface are less likely to school, and tend to scatter. There is perhaps less benefit to schooling in such cases, as there are fewer predators and little reason to attempt to obtain food at depth.

Biology

Yellowfin tuna. Photo courtesy National Park Service

Distinctive FeaturesThe yellowfin is a large tuna. Its body is strongly fusiform, and deepest under its first dorsal fin, while tapering considerably towards the caudal peduncle. Two dorsal fins are present. In adults, the second dorsal fin is very long, as is the anal fin, which is directly below the second dorsal. These fins become relatively longer in larger individuals. The pectoral fin is also long, reaching beyond the space between the dorsal fins. The caudal peduncle is very slender and includes three sets of keels. Seven to ten dorsal and ventral finlets are present. Scales are lacking behind the corselet, a band of large scales forming a circle around the body behind the head. A swim bladder is present. The eyes are small; teeth are small and conical.

Similar species occurring in the same areas as yellowfin tuna include bigeye tuna (Thunnus obesus), blackfin tuna (T. atlanticus), and albacore (T. alalunga). The bigeye tuna has shorter dorsal and anal fins than the yellowfin tuna and is generally a heavier and deeper-bodies fish. Blackfin tuna have dorsal and anal finlets that are dusky in color, rather than bright yellow with black margins as seen in the yellowfin and bigeye tunas. Albacore can be distinguished from the yellowfin with the size of the pectoral fins. The pectoral fins of the albacore are much longer than the yellowfin tuna, reaching to the second dorsal finlet in most cases. Also the caudal fin of the albacore has a white posterior edge which is lacking on the yellowfin tuna.

ColorationThe body is metallic dark blue or greenish above, while the belly and lower sides are silvery white and crossed by many vertical, interrupted lines. Perhaps most distinctly, a golden stripe runs along the side. The second dorsal and anal fins and finlets are bright yellow, and the finlets are bordered by a narrow band of black.

Yellowfin tuna. Photo courtesy U.S. Food and Drug Administration

Size, Age, and GrowthThe maximum length reported for yellowfin is 110 inches (280 cm) total length and the maximum weight is 880 lbs. (400 kg). The all-tackle record recognized by the International Game Fish Association (IGFA) is 388 lbs. 8 oz. (176.4 kg). This latter example is more indicative of the common maximum size for the species.

Food HabitsPrimary prey items include fish, cephalopods, and crustaceans. Yellowfin appear to forage rather indiscriminately for any of these items. A study by Watanabe (1958) found 37 families of fish and 8 orders of invertebrates in yellowfin stomachs. Fish species consumed by the yellowfin tuna include dolphinfish, pilchard, anchovy, flyingfish, mackerel, lancetfish, and other tunas. Other prey items are cuttlefish, squid, octopus, shrimp, lobster, and crabs.

Yellowfin are apparently sight-oriented predators, as their feeding tends to occur in surface waters during daylight. Other large fish and marine mammals compete with yellowfin for food.

ReproductionSize at maturity varies by region, and may also be different between individuals found near- and offshore. All yellowfin are reproductively mature by the time they reach a length of 47 in. (120 cm) fork length (corresponding to an age of 2-3 years), however some are mature by 20-23 in. (50-60 cm) fork length (corresponding to 12-15 months). In juvenile fishes and adults up to 55 in. (140 cm), the sex ratio is approximately 1:1. The proportion of females declines in fishes larger than this size, however the reason for this is not understood.

Reproduction occurs year-round, but is most frequent during the summer months in each hemisphere. It is believed that 79°F (26°C) is the lower temperature limit for spawning. In the tropical waters of Mexico and Central America, it has been determined that yellowfin spawn at least twice a year. Each female spawns several million eggs per year. Among tunas, larval yellowfin can be identified by the presence of a single spot of black pigment under the chin and a lack of pigment on the tail. In profile, the center of the eye is above the line of the body axis. Postlarvae and small juveniles are very difficult to distinguish from related species because these diagnostic characters become obscured. The juveniles grow quickly, weighing approximately 7.5 pounds (3.4 kg) at 18 months and 140 pounds (63.5 kg) at 4 years.

PredatorsSharks, including the bignose shark (Carcharhinus altimus), blacktip shark (Carcharhinus limbatus), and cookiecutter shark (Isistius brasiliensis), prey upon yellowfin tuna. Large bony fishes are also predators of the yellowfin tuna.